There exist different types of substrate structures on which transistors can be formed, for example, solid substrate structures or SOI substrate structures. Among transistors formed inside and on top of SOI structures, two types of transistors having different silicon layer thicknesses can be distinguished.
PDSOI (for Partially Depleted SOI) transistors are characterized by a silicon layer having a thickness which is sufficiently large, for example, greater than 30 nm, for the silicon layer never to be fully depleted in operation.
PDSOI transistors comprise a doped channel and an insulator layer (BOX) having a thickness generally in the range from 100 to 200 nm. PDSOI transistors are generally used as power transistors for analog circuits.
The forming of PDSOI transistors has the advantage of being known and controlled in the electronics industry. However, decreasing the dimensions of such components, other than the thickness of the silicon layer, while keeping a high performance level, is an issue.
FDSOI (Fully Depleted SOI) transistors are characterized by a silicon layer having a thickness sufficiently small, for example, smaller than 20 nm, for the silicon layer to be fully depleted in certain operating modes.
FDSOI transistors generally comprise a non-doped or lightly doped channel and an insulator layer (BOX) having a thickness generally in the range from 5 to 50 nm. The source/drain resistance of such transistors is greater than that of PDSOI transistors.
FDSOI transistors are preferred for logic circuits, because of their ability to operate rapidly.
An advantage of FDSOI transistors is that they have leakage currents and a power consumption lower than those of PDSOI transistors. Further, it is easier to control short-channel effects therein. However, the small dimensions may make the manufacturing difficult. Further, the small silicon thickness limits electric currents.
In common integrated circuits, co-integrations of bulk transistors and of SOI transistors of a given type can be encountered.